The present invention relates to an illumination apparatus having an irradiation source and a light guide coupled thereto. The light guide is flexible and positioned within a metallic protection tube which is flexible as well. The light guide includes a temperature monitoring mechanism which monitors the temperature along the complete length of the metallic flexible protection tube. In case the local threshold temperature is exceeded anywhere on the outer surface of the metallic protection tube, the radiation source is switched off with a delay of only a few minute. A temperature monitoring mechanism of this kind has become necessary because market-available radiation sources, such as video projector lamps, have become more and more powerful, and those radiation sources are coupled to flexible light guides, particularly to liquid core light guides as described in German Patent No. DE 42 33 087, for example.
For instance, it is possible to transmit, by means of a video projector lamp having a power of 200 W (e.g. Osram™-P-VIP200) or a video projector lamp having an output power of 330 W (e.g. Philips™-TOP-UHP 330 W) with an integrated ellipsoidal reflector, up to 40 W radiation power in the visible spectrum as an output from a liquid core light guide having a light active aperture of only 5 mm diameter and a total length of a few meters. Such high radiation output powers from a flexible light guide had so far been possible by means of laser radiation sources only.
What is industrially needed is an apparatus with a light guide for illumination in the visible range having an output radiation power which is as high as possible. For example, a 200 W video projector lamp of the type Osram™ P-VIP200 and a liquid core light guide of 10 m length and 5 mm light active diameter are used for three-dimensional optical digitization and measurement. In this example, the radiation output power from the light guide is 20 W!
Because the core of the light guide consists of an aqueous solution having a boiling point which is only slightly above 100° C., even small contaminations of the liquid core or insufficient filtering of the infrared portions of the radiation emitted from the radiation source or a mechanical deforming of the liquid core light guide by exertion of outer forces involve the danger of bubbles being formed in the liquid or a local overheating of the liquid core light guide and a quickly escalating temperature rise of the outer envelope which can damage the liquid core light guide and cause secondary damages. A monitoring of the outer temperature of the liquid core light guide and a possibility for switching off the radiation source have therefore become absolutely necessary.
FIG. 1 shows the temperature increase of the outer tube of a liquid core light guide (in this case a corrugated tube or a wrapped or spiralled tube made of stainless steel) as a function of time for a liquid core light guide having a light active diameter of 5 mm and a radiation output power of about 20 W, wherein a bubble of 3 mm diameter has artificially been set in the liquid. It is clear from this example that the radiation source (in this case a Philips™ video projector lamp having an electrical power of 330 W) should be switched off after 6 to 7 minutes at the latest, because the outer cover or envelope of the liquid core light guide (a flexible corrugated tube made of stainless steel) has already reached a temperature of more than 130° C.
FIG. 1 also shows that the radiation output power of the liquid core light guide decreases rapidly when the temperature of the outer corrugated tube of the liquid core light guide reaches values of more than 170 to 180° C. It is not sufficient to monitor the liquid core light guide by means of a beam splitter, because the transmission of the liquid core light guide at first decreases only slowly even though the outer temperature of the light guide protection tube has already reached a value of more than 120° C.